Residues and Routes of Exposure of Insecticides in Turfgrass for Control of Fall Armyworm Larvae (Lepidoptera: Noctuidae)

Abstract Fall armyworm, Spodoptera frugiperda (J.E. Smith), is a generalist pest of food crops and turfgrasses. Insecticides such as diamides provide longer residual control of some grass-feeding caterpillars relative to pyrethroids. The objective was to compare the responses of fall armyworm larvae to residues of commonly used insecticides on hybrid bermudagrass (Cynodon dactylon (L.) × C. transvaalensis Burtt-Davy). A field experiment determined mortality of free-ranging fall armyworms exposed for 1–3 h on turfgrass at 1, 7, 14, 28, and 42 d after a foliar application. A laboratory experiment determined the mortality of larvae fed grass clippings harvested from treated plots at 1, 3, 7, 14, 28, and 42 d after application. Larvae crawled similar or greater distances across bermudagrass treated with bifenthrin and permethrin relative to nontreated grasses. After crawling on bermudagrass treated with chlorantraniliprole or cyantraniliprole 14 d after application, larval mortality was ≥77%. Mortality of larvae fed pyrethroid-treated clippings was ≤40% and no different from control larvae regardless of residue age. Larvae fed bermudagrass with 1 to 7 d old residues of cyantraniliprole or indoxacarb resulted in significantly greater mortality than larvae fed nontreated clippings. No residues of cyantraniliprole or indoxacarb older than 7 d resulted in mortality significantly greater than larvae fed nontreated clippings. Larvae fed chlorantraniliprole-treated clippings of all residue ages produced larval mortality greater than larvae fed nontreated clippings. These experiments support previous reports of extended residual control from chlorantraniliprole against other grass-feeding caterpillar species. The diamides protect turfgrass from larvae better than pyrethroids.